For signal transmission in an information apparatus such as a laptop personal computer, cellular phone, and video camera, a differential signal transmission system is often employed from a point of a measure to counter electromagnetic wave interference. A differential signal transmission system is a system to transmit a positive signal and negative signal with two signal conductors and the difference between both signals is treated as a signal value. In the differential signal transmission system, because directions of current in two signal conductors are mutually reversed, magnetic fields produced by the signals cancel out each other outside of the conductors. The smaller the distance between two signal conductors, the more effective the cancellation.
For a cable for differential signal transmission in the above-mentioned information apparatus, a two-core parallel shield cable 71 as shown in FIG. 7 is known. The shield cable 71 has two insulated wires 2 for signal transmission arranged in parallel, a first shield layer 75 and second shield layer 6 formed 5 with conducting wires integrally and spirally wound on the wires, and a sheath 8 over the shield layers. As occasion demands, a third shield layer 7 consisting of a metal tape, etc. is provided between the shielding layers 75 and 6 and the sheath 8. The shield cable 71 can be made more easily than a shield cable in which a shield layer is formed with a braided conducting wire and has an advantage in terms of cost in the case having a small diameter.
The insulated wire 2 has a structure in which a signal conductor 3 having an outer diameter of 0.09 mm and consisting of seven stranded tin plated copper alloy wires each having an outer diameter of 0.03 mm is covered with a fluoric resin insulator 4 such that the outer diameter of the insulated wire 2 is 0.21±0.03 mm. The shield layer 75 is formed by spirally winding, at a pitch of 5 to 7 mm, about 33 to 43 tin plated copper alloy conducting wires, which are the same wires as those used for the signal conductor 3, each having an outer diameter of 0.03 mm.
In a case of a cable having only the shield layer 75, when it is bent or twisted, a gap may arise between the conducting wires, resulting in shortage of shielding effect, and therefore the shield layer 6 is formed on the shield layer 75 so that the shielding effect can be assured. The shield layer 6 is formed by spirally winding, at a pitch of 5 to 7 mm, 38-48 conducting wires, which are the same wires as those used for the shield layer 75. Usually the shield layer 6 is formed by winding the conducting wires in an opposite winding direction relative to the winding direction of the shield layer 75. The sheath 8 is formed with a polyester tape wound on the shield layer.
With one or more shield cables 71 having an above-described structure, a wiring between a main body portion and a liquid crystal display is provided in an information apparatus. In this case, one or more shield cables 71 are wired through a connection portion having a hinging mechanism for opening and closing the display.
Repeated opening and closing of the display causes a conducting wire of the shield layer 75 to break and the broken wire to stick to the insulator 4 of insulated wire 2 so that the broken wire and the signal conductor 3 short-circuit. In addition, in case a plurality of shield cables 71 are bundled, repeated opening and closing of the display causes the signal conductor 3 to disconnect.